The early history of lenses is unknown. In 1845 an archeologist uncovered in what is now Iraq an ancient rock crystal ground to form a small convex lens, but there is no evidence that lenses were widely known or used in ancient times. An early investigation of the principles of lenses was made in the 11th century by Alhazen, a Persian physicist. Spectacles with convex lenses were in common use both in Europe and in China as early as the 13th century.
Zacharias Janssen, a Dutch optician, is credited with combining lenses to make a compound microscope about 1590. Galileo improved the telescope in 1609. The art of design and manufacture of lenses has progressed steadily since that time.
Ophthalmic Lenses are designed to refract light so that it reaches the retina. It is necessary for light rays to focus on each retina, in the back of our eyes, in order for us to see. When the eye's own optical system cannot refract light onto the retina, ophthalmic lenses are needed.
The main job of the ophthalmic lens is to manipulate light. Lenses are used for many reasons. The main reasons they are prescribed is for safety, vision, and comfort. Different materials are used for specific purposes and visual needs. Most ophthalmic lenses refract light rays to reach the retina, reflect uncomfortable or dangerous light rays, reflect annoying glare, absorb light for comfort or safety, and transmit light for better vision.
The most common reason lenses are prescribed is for vision. Three factors determine the prescription in a lens. They are the material (index of refraction), thickness, and curvature. In theory, lenses are considered as being prisms attached base-to-base or apex-to-apex. A convex lens (prisms base-to-base) magnifies images, and is used for the correction of Hyperopia or Presbyopia. A concave lens (apex-to-apex) minifies images, and is used for the correction of Myopia. Cylinder (toric) lenses are also used for the correction of astigmatism, and have different powers in different zones of the lens.
The design and production of lenses is a complex art and science. Glass and Plastic-type lenses, with the exception of polycarbonate, are made from a molding process. First, the lens designer calculates the best curvatures necessary for superior optical quality. Then, a computerized program attached to a digital file and lathe cuts the inside/top surface of the mold. This could be a single curvature for single vision lenses, or multiple curves for an aspheric lens or a progressive addition lens. Then, the back of the mold is typically attached and liquid plastic (monomers and polymers) are added. After the annealing and cooling process is complete, the lens is ready to be surfaced. Surfacing a lens means cutting curves into the back side to create the prescribed numbers, or spectacle prescription. In simple spherical curves, a steeper curve yields a stronger prescription and a flatter curve yield a weaker prescription. Once polished, the finished lens is ready to be edged for the shape of the frame. Many coatings and filters can be added to the lens to manipulate light in different ways, such as ultraviolet filters and scratch resistant coatings.
In milling the lens, typically a disk-like tool is used having a series of cutting edges located around the outer edge of the disk. These cutting edges can be made from a diamond or a diamond like substance and are typically brazed onto the disk. An example is Satisloh's 12 Blade fixed PCD insert milling wheel, model 92-009-346. However, to replace the cutting edges on this milling wheel, each cutting bit needs to be heated such that the brazing liquefies thereby allowing the cutting edge to be removed from the tool body. This process is both costly and time consuming.
Another drawback to using a disk with fixed cutting edges is the limited amount of the cutting edge that can be used before the cutting edges need to be sharpened or replaced when they can no longer be sharpened. Modern lens generators have a small footprint, which limits the amount the cutting tool may move relative to the lens. This limited amount of movement results in only a small portion of the cutting edge being used before the tool needs to be sharpened or replaced, which adds additional cost and time to the lens production process. To overcome this inefficiency in the production process, tools having quick replacement cutting edges have been developed. This design allows for the cutting edges to be rotated in place thereby moving a fresh and sharp section of the cutting edge to the point that contacts the lens. However, even these new designs have limitations. For instance, Satisloh's T66 Cutting Wheel with PCD inserts, model number 92-002-738, replaces typical fixed inserts with 8 replaceable inserts. This newer design provides an improvement over earlier designs, yet it still has drawbacks. For example, Satisloh's design only allows the cutting edges to be rotated to a clean cutting position up to three (3) times before the cutting edge must be removed and sharpened as suggested by the manufacturer. In addition, the cutting edges may only be sharpened up to 3 times, depending on the condition of the cutting edges, further limiting the cost and time advantages of this design.
Another example is Mapal's cutting wheel. This wheel also has several drawbacks. The cutting edge inserts are located off the center bore of the flute into which the cutting disk mounts. Variations in the bore of each flute results in inaccurate location of the cutting edges with regards to generating an effective and consistent radius during rotation. The cutting disks are secured with a screw and reverse clamp thread plate where the cutting disk is held in place with no more than three (3) threads on average, resulting in the cutting disk insert possibly becoming loose during operation. During use, the cutting disk insert may become stuck in the flute. Current cutting tools lack a release mechanism allowing for the safe release of a cutting disk in the event it becomes stuck. Due to the extreme sharp nature of the cutting edges, a user's hand or fingers could easily be cut while attempting to remove a stuck cutting disk insert from the flute. The clamp plates and center location bores are easily damaged during clamping and tool setting. Finally, clearance between flutes makes assembly and disassembly difficult and may result in injury to the user or damage to the cutting disk during insertion or removal of the cutting disk.
What is needed in the industry is a cutting wheel having replaceable cutting edges. A further benefit is a cutting wheel where the cutting edges are positionable to more than three (3) positions thereby extending the life of the tool as well as minimizing the costs associated with removal and replacement of the cutting tool as well as the costs associated with the sharpening of the edges themselves.